Magnetic memory devices, such as magnetic random access memory (MRAM) devices, use magnetic memory cells to store information. Information is stored in a magnetic memory cell as the orientation of the magnetization of a free layer in the magnetic memory cell as compared to the orientation of the magnetization of a fixed or pinned layer in the memory cell. The magnetization of the free layer may be oriented parallel or anti-parallel to the fixed layer, representing either a logic “0” or a logic “1.” One type of memory cell, a magnetic tunnel junction (MTJ), comprises a free layer and a fixed layer separated by a thin dielectric barrier (a tunnel barrier), which typically comprises aluminum oxide. The resistance of the memory cell depends on the direction of magnetization of the free layer relative to the direction of magnetization of the fixed layer. Thus, the state of the cell can be sensed by measuring its resistance.
Reactive ion etching (RIE) is commonly used in MRAM processing as a means of patterning MTJ features. In RIE, reactive gases are ionized and accelerated towards the substrate. These reactive gases play two roles. They sputter the material from the surface, as well as chemically react with the material, thereby producing reaction products that are volatile and can be pumped away.
Because the etching medium is a flux of ions directed towards the substrate, RIE is predominantly anisotropic, meaning that etching occurs preferentially in the direction normal to the substrate. This translates into an etch rate on surfaces perpendicular to the substrate that is much lower than the etch rate on surfaces parallel to the substrate. As a result, redeposition of etching byproducts may occur, particularly on the sidewalls of vertical features where the etch rate is relatively low.
Such byproduct redeposition is especially problematic in producing MRAM circuitry. The etching byproducts formed when etching MTJ features are extremely difficult or impractical to remove without using methods that also cause harm to the sensitive film stack that makes up the etched device itself. As a result, redeposition of etching byproducts remains a major source of yield reduction in MRAM processing.
It is known that tapering an etched feature can result in increased sputter yield at the feature's sidewalls, and, thereby, reduce redeposition. The difficulty, however, lies in finding a reliable and reproducible way of forming such a taper. Two techniques are known in the art. In a first technique, the RIE chemistry is adjusted by balancing the rates of anisotropic physical sputtering, isotropic chemical etching, and byproduct redeposition. However this balancing process is complex, and the balance of these three components is highly sensitive to the condition of the etch tool.
In a second technique, a masking layer is first deposited on the film stack and patterned such that the masking layer can act as a hard mask during the etching of the underlying film stack. The masking layer is then physically sputtered so that its corners are eroded, thereby creating a taper in the masking layer which can subsequently be translated into the film stack. Nevertheless, because of the possibility of damage to the underlying film stack, the physical sputtering of the masking layer is usually limited. This frequently means that lower reaches of the masking layer cannot be tapered sufficiently. As a result, the redeposition of etching byproducts is frequently still problematic when subsequently etching the remainder of the film stack.
Accordingly, there is a need for a method of forming MTJ features in MRAM integrated circuits that is both more reliable and more reproducible than those currently known in the art, and does not suffer from one or more problems exhibited by conventional MTJ processing methodologies.